Hearing aid users have been reported to have poorer ability to localize sound sources when wearing their hearing aids than without their hearing aids. This represents a serious problem for the mild-to-moderate hearing impaired population.
Furthermore, hearing aids typically reproduce sound in such a way that the user perceives sound sources to be localized inside the head. The sound is said to be internalized rather than being externalized. A common complaint for hearing aid users when referring to the “hearing speech in noise problem” is that it is very hard to follow anything that is being said even though the signal to noise ratio (SNR) should be sufficient to provide the required speech intelligibility. A significant contributor to this fact is that the hearing aid reproduces an internalized sound field. This adds to the cognitive loading of the hearing aid user and may result in listening fatigue and ultimately that the user removes the hearing aid(s).
Thus, there is a need for a new hearing aid with improved localization of sound sources, i.e. the new hearing aid preserves information of the directions and distances of respective sound sources in the sound environment with relation to the orientation of the head of the wearer of the hearing aid.
Human beings detect and localize sound sources in three-dimensional space by means of the human binaural sound localization capability.
The input to the hearing consists of two signals, namely the sound pressures at each of the eardrums, in the following termed the binaural sound signals. Thus, if sound pressures at the eardrums that would have been generated by a given spatial sound field are accurately reproduced at the eardrums, the human auditory system will not be able to distinguish the reproduced sound from the actual sound generated by the spatial sound field itself.
It is not fully known how the human auditory system extracts information about distance and direction to a sound source, but it is known that the human auditory system uses a number of cues in this determination. Among the cues are spectral cues, reverberation cues, interaural time differences (ITD), interaural phase differences (IPD) and interaural level differences (ILD).
The transmission of a sound wave from a sound source positioned at a given direction and distance in relation to the left and right ears of the listener is described in terms of two transfer functions, one for the left ear and one for the right ear, that include any linear distortion, such as coloration, interaural time differences and interaural spectral differences. Such a set of two transfer functions, one for the left ear and one for the right ear, is called a Head-Related Transfer Function (HRTF). Each transfer function of the HRTF is defined as the ratio between a sound pressure p generated by a plane wave at a specific point in or close to the appertaining ear canal (pL in the left ear canal and pR in the right ear canal) in relation to a reference. The reference traditionally chosen is the sound pressure pI that would have been generated by a plane wave at a position right in the middle of the head with the listener absent.
The HRTF contains all information relating to the sound transmission to the ears of the listener, including diffraction around the head, reflections from shoulders, reflections in the ear canal, etc., and therefore, the HRTF varies from individual to individual.
In the following, one of the transfer functions of the HRTF will also be termed the HRTF for convenience.
The hearing aid related transfer function is defined similar to a HRTF, namely as the ratio between a sound pressure p generated by the hearing aid at a specific point in the appertaining ear canal in response to a plane wave and a reference. The reference traditionally chosen is the sound pressure pI that would have been generated by a plane wave at a position right in the middle of the head with the listener absent.
The HRTF changes with direction and distance of the sound source in relation to the ears of the listener. It is possible to measure the HRTF for any direction and distance and simulate the HRTF, e.g. electronically, e.g. by filters. If such filters are inserted in the signal path between a playback unit, such as a tape recorder, and headphones used by a listener, the listener will achieve the perception that the sounds generated by the headphones originate from a sound source positioned at the distance and in the direction as defined by the transfer functions of the filters simulating the HRTF in question, because of the true reproduction of the sound pressures in the ears.
Binaural processing by the brain, when interpreting the spatially encoded information, results in several positive effects, namely better signal-to-noise ratio (SNR); direction of arrival (DOA) estimation; depth/distance perception and synergy between the visual and auditory systems.
The complex shape of the ear is a major contributor to the individual spatial-spectral cues (ITD, ILD and spectral cues) of a listener. Devices which pick up sound behind the ear will, hence, be at a disadvantage in reproducing the HRTF since much of the spectral detail will be lost or heavily distorted.
This is exemplified in FIGS. 1 and 2 where the angular frequency spectrum of an open ear, i.e. non-occluded, measurement is shown in FIG. 1 for comparison with FIG. 2 showing the corresponding measurement on the front microphone on a behind the ear device (BTE) using the same ear. The open ear spectrum shown in FIG. 1 is rich in detail whereas the BTE result shown in FIG. 2 is much more blurred and much of the spectral detail is lost.